Flow measuring devices are differentiated according to various criteria. The most widely used differentiating criterion is that based on measuring principle. Correspondingly, e.g. Coriolis flow measuring devices, ultrasonic, flow measuring devices, thermal, flow measuring devices, vortex, flow measuring devices, magneto-inductive flow measuring devices, SAW (surface acoustic wave) flow measuring devices, V-cone flow measuring devices and suspended body flow measuring devices are known. Corresponding flow measuring devices are available from the applicant or others. In the present case, of concern is especially a magneto-inductive flow measuring device.
A further differentiating feature concerns whether the flow measuring device has a measuring tube or whether the flow measuring device can be placed or built on a conventional pipeline, or process line, without interrupting the flow of medium within the pipeline for the installation of the flow measuring device. Typical ultrasonic measuring devices of this type are so-called clamp-on ultrasonic, flow measuring devices. Flow measuring devices with measuring tubes are known as inline flow measuring devices.
For quite some time, inline flow measuring devices have been known, which are built with measuring tubes having a reduced cross section. A corresponding measuring tube is shown, for example, in U.S. Pat. No. 4,899,592.
Predominantly, concepts for measuring tubes with reduced cross section utilize measuring tubes involving a metal tube and an internally situated, plastic liner. For a uniform liner thickness, the liner must either especially be cast or inserted already preformed into the measuring tube on both sides and then centrally welded. This leads to a higher manufacturing effort and, in given cases, to measurement deviations.
Known additionally are plastic tubes, e.g. of polyethylene or polyamide, which can be used as measuring tubes in magneto-inductive flow measuring devices. Fundamentally, the application of plastic tubes is, however, limited, since they expand or contract with change of the pressure of the medium. A change of the diameter of a measuring tube causes a measurement error. In the case of a measuring tube with reduced cross section, the widening tendency would be even greater in the region of the reduced cross section.
U.S. Pat. No. 8,286,503 provides for solving this problem stiffening ribs, which enable a stabilizing of the measuring tube cross section.